A hydrogen-gas discharge lamp can comprise a bulb composed of glass and having a radiation-emission window through which an emitted radiation emerges from the lamp. The lamp can be provided with a material serving as a reservoir for hydrogen and/or deuterium as well as a reservoir for hydrogen and/or deuterium as well as a getter for gaseous components which might interfere with the purity of the emission. Means is provided to excite the hydrogen and/or deuterium which is liberated in the bulb.
Hydrogen gas discharge lamps of this type are known. The hydrogen reservoir/getter combination was formed by uranium in such systems (see Dieke, G. H. and Cunningham, S. P. A New Type of Hydrogen Discharge Tube, J. Opt. Soc. America, 1952, 42, 187-189).
The use of a hydrogen reservoir/getter combination is advantageous on two grounds. Firstly, a Lyman-.alpha. lamp (a hydrogen discharge lamp with the principal emission being the Lyman-.alpha. line of 121.56 nm as a line source) must have a relatively low hydrogen partial pressure of the order of magnitude of 1 Pa since with higher pressures, the emission lines of molecular hydrogen are more pronounced than the Lyman-.alpha. line. Because of the diffusion of hydrogen through glass, the filling of the bulb with hydrogen at such low partial pressures can result in a lamp with a very limited life span. For this reason it has been found to be advantageous to liberate the hydrogen progressively by thermal means from a metal hydride forming the hydrogen reservoir. The partial pressure of the hydrogen is then determined by the temperature of the hydride. This can be calculated from the van 'T Hoff equation ##EQU1##
In this equation p represents the hydrogen partial pressure and T the absolute temperature. A and B are parameters specific to the material. For example, typical values for A and B for uranium (p in Pa) are: A=4366 and B=11.26 (Isotope Effect in Dissociation of Uranium Hydride, Nuclear Science and Technol., (1979), 16, 690-696).
To obtain the hydrogen partial pressure for operation of a Lyman-.alpha. lamp, a temperature of about 100.degree. C. should be used.
The second ground for utilizing a hydrogen reservoir/getter combination is the spectral purity of the lamp. Foreign atoms or molecules which may remain trapped in the bulb or penetrate into the lamp by diffusion also emit electromagnetic radiation at wavelengths characteristic of the respective atoms. To ensure high spectral purity, these atoms or molecules must be removed from the gas space. This can be achieved with the getter characteristics of uranium. Uranium reacts with a variety of foreign atoms to bind them chemically so that they no longer are involved in the emission process in the lamp.
Use of uranium as a hydrogen reservoir, however, has a number of drawbacks which are intrinsic to the characteristics of this material:
Uranium is radioactive
Finely divided uranium is pyrophoric, i.e. it ignites in contact with air.
The use of uranium requires observance of environmental laws, requires special permits and requires special care in handling.
Uranium is poisonous.
Because of the short useful life of a Lyman-.alpha. lamp (several tens of hours), the use of uranium in such lamps creates disposal problems.
Safe handling of uranium requires the use of inconvenient glove boxes.
The availability of uranium can be a problem.